Gas generating composition

ABSTRACT

A gas generating composition having a low combustion temperature and a low heat of combustion at the time of burning and therefore enabling downsizing of gas generators themselves, which comprises as the essential components nitroguanidine and an oxidizing agent comprising (a) nitrates or nitrites of alkali metals or alkaline earth metals, (b) oxides or multiple oxides of metals selected from among copper, cobalt, iron, manganese, nickel, zinc, molybdenum and bismuth, or a mixture of the components (a) and (b).

TECHNICAL FIELD TO WHICH THE INVENTION BELONGS

The present invention relates to a gas generating composition whichevolves a working gas in the air bag system to be mounted inautomobiles, aircraft or the like for the protection of the human body.

PRIOR ART

Sodium azide is known to the public as the gas generating agent to beused in the air bag system for automobiles, aircraft or the like.Further, gas generating compositions comprising sodium azide are notparticularly problematic in their burning characteristics and thereforeare widely put to practical use. However, sodium azide has unfavorabledisadvantages. For example, many patents in this field have pointed outvarious problems such as dangerous decompositional explosion, formationof explosive compounds by the reaction with heavy metals, environmentalpollution caused by mass disposal of the waste and so on.

Meanwhile, investigations are in progress on the substitutes for sodiumazide to solve these problems. For example, gas generating compositionscomprising transition metal complexes of tetrazole or triazole aredisclosed in JP-B 6-57629; ones comprising triaminoguanidine nitrate inJP-A 5-254977; ones comprising carbohydrazide in JP-A 6-239683; and onescomprising cellulose acetate and nitrogenous nonmetallic compounds suchas nitroguanidine in JP-A 7-61885.

Further, U.S. Pat. No. 5,125,684 discloses the use of nitroguanidine asthe energetic component to be made coexistent with 15 to 30%cellulose-based binder; and WO-A 96/25375 (published on Aug. 22, 1996)discloses combinations of nitroguanidine with silicon oxide or aluminumoxide.

When the burning of a nitrogenous organic compound is conducted by theuse a stoichiometric amount of an oxidizing agent, i.e., such an amountto generate enough oxygen to complete the burning of carbon, hydrogenand other elements constituting the compound, the heat of combustion andthe combustion temperature are disadvantageously generally higher thanthose found in the burning of azide compounds. In addition to theperformance of a gas generating agent, it is essential for the inflatersystem for air bags that the system itself has a size that does nothinder the ordinary driving of an automobile. When a gas generatingagent having a high combustion temperature and a high heat of combustionis used, however, the use of additional parts for heat removal isunavoidable in designing a gas generator, which makes it impossible todownsize a gas generator. In short, it is most preferable that a gasgenerating agent have a low combustion temperature and a low heat ofcombustion at the time of burning. Accordingly, it cannot be said thatthe known gas generating compositions described above are satisfactoryin applicability to air bag systems.

DISCLOSURE OF THE INVENTION

The inventors of the present invention have intensively studied to solvethe above problems, and the present invention has been accomplished as aresult of the studies.

The present invention relates to a gas generating composition whichcomprises as the essential components nitroguanidine and an oxidizingagent comprising the following component (a) or (b) or a mixture of thecomponents (a) and (b):

(a) nitrates or nitrites of alkali metals or alkaline earth metals, and

(b) an oxide or a multiple oxide of a metal(s) selected from copper,cobalt, iron, manganese, nickel, zinc, molybdenum and bismuth.

The composition is preferably a combination of nitroguanidine with thecomponent (b), or with the components (a) and (b).

The composition preferably comprises 20 to 80% by weight ofnitroguanidine and 80 to 20% by weight of the oxidizing agent, and stillpreferably comprises 25 to 40% by weight of nitroguanidine and 75 to 60%by weight of the oxidizing agent.

The composition may further contain at most 5% by weight of a binder orbinders based on the total weight of the composition.

The oxidizing agent is preferably one wherein the component (a) is analkaline earth metal nitrate, more preferably strontium nitrate, onewherein the component (a) is an alkali metal nitrate, still preferablypotassium nitrate, one wherein the component (b) is an oxide, morepreferably copper oxide, or one comprising strontium nitrate and copperoxide.

The modes for carrying out the invention will now be described indetail.

The content of nitroguanidine in the gas generating composition of thepresent invention is preferably 20 to 80% by weight, more preferably 20to 60% by weight, though it depends on the kind of the oxidizing agentand the oxygen balance. The content is most desirably 25 to 40% byweight because, in such a case, a gas generating composition comprisinga copper oxide as the oxidizing agent exhibits a combustion temperatureof 2200° K or below and a gas yield of 1.2 mol/100 g or above.

In the present invention, the above component (a) or (b) or a mixture ofboth is used as the oxidizing agent. Specific examples of (a) nitritesor nitrates of alkali metals or alkaline earth metals include alkalimetal or alkaline earth metal salts of nitric acid, such as sodiumnitrate, potassium nitrate, magnesium nitrate, strontium nitrate; andalkali metal or alkaline earth metal salts of nitrous acid, such assodium nitrite, potassium nitrite, magnesium nitrite and strontiumnitrite. On the other hand, specific examples of (b) oxides or multipleoxides of metals selected from copper, cobalt, iron, manganese, nickel,zinc, molybdenum and bismuth include CuO, Cu₂O, Co₂O₃, CoO, Co₃O₄,Fe₂O₃, FeO, Fe₃O4, MnO₂, Mn₂O₃, Mn₃O₄, NiO, ZnO, MoO₃, CoMoO₄, Bi₂MoO₆and Bi₂O₃. Among these oxidizing agents, copper oxides are the mostdesirable.

The oxidizing agent to be used in the present invention may be any oneselected from among the above compounds or any combination of two ormore members selected from among them. The content of the oxidizingagent in the gas generating composition is preferably 80 to 20% byweight, still preferably 80 to 40% by weight. The content is mostdesirably 75 to 60% by weight because, in such a case, a gas generatingcomposition comprising a copper oxide as the oxidizing agent exhibits acombustion temperature of 2200° K or below and a gas yield of 1.2mol/100 g or above.

The gas generating composition of the present invention may furthercontain a binder or binders. The binder to be used in the presentinvention includes inorganic ones such as silica, alumina and molybdenumdisulfide; and organic ones such as microcrystalline cellulose, POVALand high-molecular-weight oligomers. It is preferable that the bindercontent of the composition be 5% by weight or below.

The gas generating composition of the present invention comprisingnitroguanidine and the above oxidizing agent generally exhibits alowered combustion temperature and a lowered total heat of combustion ascompared with those of the gas generating compositions of the prior art.In particular, a composition comprising nitroguanidine and CuO isextremely excellent in these respects, thus being the most importantcombination. Further, this composition can change in the burningvelocity and the pressure exponent by controlling the oxygen balance.The term “pressure exponent” as used in this description refers to theexponent “n” in the formula: burning velocity r (mm/sec)=a×p^(n)(wherein a is a constant dependent on the constituents of a gasgenerating composition and initial temperature; and p is pressure(kgf/cm²) of measurement). The above composition has the property that napproaches O as the oxygen balance shifts to the plus side, which isparticularly preferable from the standpoint of the stability ofcombustion.

The gas generating composition of the present invention is preferablyprepared by mixing the components in a powdery state, and the mixing canalso be conducted according to a wet process in the presence of water orthe like, at need. Prior to the use, the composition may, if necessary,be molded into a suitable shape such as granule, pellet or disk.Further, a composition having a low burning velocity may be molded byextrusion prior to the use.

The gas generating composition of the present invention is particularlyuseful for the air bag system to be mounted in automobiles, aircraft orthe like for the protection of the human body.

The nitroguanidine contained in the gas generating composition of thepresent invention exhibits high long-term stability required of the airbag system and has excellent burning characteristics.

EXAMPLE

The present invention will now be described specifically by referring tothe following Examples and Comparative Examples, though the presentinvention is not limited by them. In the following Examples andComparative Examples, the compositions were pelletized in an ordinarymanner before being examined.

Examples 1 to 5 and Comparative Examples 1 to 7

Table 1 shows the theoretical combustion temperatures ofnitroguanidine-containing gas generating compositions according to thepresent invention. The Comparative Examples show the theoreticalcombustion temperatures of gas generating compositions (ComparativeExamples 1, 2) comprising transition metal complexes of 5-aminotetrazole(5-AT) (disclosed in JP-B 6-57629), gas generating composition(Comparative Example 3) comprising triaminoguanidine nitrate (disclosedin JP-A 5-254977), gas generating composition (Comparative Example 4)containing carbohydrazide (disclosed in JP-A 6-239683), and gasgenerating compositions (Comparative Examples 5, 6 and 7) comprisingcellulose acetate and nitrogenous nonmetallic compounds (disclosed inJP-A 7-61885).

Examples 6 to 14

A gas generating composition comprising nitroguanidine and CuO couldchange in combustion temperature, burning velocity, density of pelletsof the gas generating composition and gas yield, by changing the mixingratio of the components. The data are given in Table 2. Each burningvelocity was determined under a pressure of 70 kgf/cm².

As will be understood from the results, the gas generating compositionsof the present invention are superior to those of the prior art,particularly in combustion temperature, which paves the way to downsizegas generators and apply the technology to air bag systems.

TABLE 1 Combustion Compn. (wt %) temp. (° K.) Ex. 1 nitroguanidine/KNO₃(56.3/43.7) 2200 2 nitroguanidine/Sr(NO₃)₂/CuO (40.3/19.2/40.5) 2091 3nitroguanidine/CuO (39.5/60.5) 2043 4 nitroguanidine/KNO₃/Al₂O₃(55.2/42.8/2.0) 2172 5 nitroguanidine/CuO/cellulose (32.8/64.7/2.5) 1928Comp. Ex. 1 Zn(5-AT)₂/Sr(NO₃)₂ (44.0/56.0) 2411 2[Cu(S-AT)_(2.1)/2H₂O]/Sr(NO₃)₂ (42/58) 2390 3 triaminoguanidine nitrate/(57.9/42.1) 2911 KClO₄ 4 carbohydrazide/KClO₄/CaO (39/61/10) 2825 5cellulose acetate/triacetin/ (8/2/55/35) 2834 KClO₄/nitroguanidine 6cellulose acetate/triacetin/ (8/4/57/31) 2893 KClO₄/triaminoguanidinenitrate 7 cellulose acetate/triacetin/ (10/5/65/20) 2928KClO₄/5-aminotetrazole

TABLE 2 Gas yield Com- (mol/100 bustion Burning g of gas (wt temp.velocity Density generating Compn. %) (° K.) (mm/s) (g/cm³) compn.) Ex. 6 nitroguanidine/CuO (39.5/ 2043 4.3 2.54 1.90 60.5)  7nitroguanidine/CuO (38/ 1992 4.9 2.56 1.83 62)  8 nitroguanidine/CuO(36/ 1922 5.8 2.58 1.73 64)  9 nitroguanidine/CuO (34/ 1850 6.9 2.631.63 66) 10 nitroguanidine/CuO (32/ 1774 8.8 2.71 1.54 68) 11nitroguanidine/CuO (30/ 1695 9.1 2.76 1.44 70) 12 nitroguanidine/CuO(28/ 1604 10.6 2.83 1.35 72) 13 nitroguanidine/CuO (26/ 1526 11.0 2.941.25 74) 14 nitroguanidine/CuO (24.6/ 1517 9.5 2.94 1.18 75.4)

What is claimed is:
 1. A gas generating composition comprisingnitroguanidine, a binder selected from the group consisting of alumina,molybdenum disulfide, microcrystalline cellulose, POVAL andhigh-molecular-weight oligomers, and an oxidizing agent having thefollowing components (a) or (b) or a mixture of the components (a) and(b): (a) nitrates or nitrites of alkali metals or alkaline earth metals,and (b) oxides or multiple oxides of metals selected from the groupconsisting of copper, cobalt, iron, manganese, nickel, zinc, molybdenumand bismuth.
 2. The composition according to claim 1, which comprises 20to 80% by weight of nitroguanidine and 80 to 20% by weight of theoxidizing agent.
 3. The composition according to claim 1, whichcomprises 25 to 40% by weight of nitroguanidine and 65 to 75% by weightof the oxidizing agent.
 4. The composition according to claim 1, whereinthe component (a) is an alkaline earth metal nitrate.
 5. The compositionaccording to claim 4, wherein the component (a) is strontium nitrate. 6.The composition according to claim 1, wherein the component (a) is analkali metal nitrate.
 7. The composition according to claim 6, whereinthe component (a) is potassium nitrate.
 8. The composition according toclaim 1, wherein the component (b) is an oxide.
 9. The compositionaccording to claim 8, wherein the component (b) is a copper oxide. 10.The composition according to claim 1, wherein the oxidizing agentcomprises strontium nitrate and a copper oxide.
 11. The compositionaccording to claim 1, wherein the amount of the binder is 5% by weightor below based on the total amount of the composition.
 12. Thecomposition according to claim 1, wherein the binder is selected fromthe group consisting of silica, alumina, molybdenum disulfide,microcrystalline cellulose, POVAL and high-molecular-weight oligomers.13. The composition according to claim 1, wherein the oxidizing agent isthe component (a).
 14. The composition according to claim 1, wherein theoxidizing agent is the component (b).
 15. The composition according toclaim 1, wherein the oxidizing agent is a mixture of the components (a)and (b).